60 



about in two '.vays : by multiplication of the cells already 

 incorpoi-ated in the epithelium cf the tentacle, and by im- 

 migration of cells from the thick ectodermal pad at the 

 base. The cartilaginous tissi^e coia])osing this pad is pecu- 

 liar in character. The cell walls are aLnost or quite 

 obliterated, and the gelatinous substance contains the con- 

 cretions already mentioned, v/hich give tho tissue something 

 of the nature of a cartilaginous encasement for the nerve 

 ring lying immediately beneath it. These concretions are 

 not very different in appearance from the concretions of 

 the sense-organs - the "otoliths" as they are called, with, 

 however, quite inadequate evidence that they function as 

 auditory organs. They are solid masses of hard substance, 

 partly calcareous and partly organic. They are not made up 

 of concentric lamellae such as give the concretion in the 

 sensory capsule its onion-like layers. In this whole group 

 of mediisae the older tentacles are left stranded as it were 

 by the growth of the margin of the umbrella beyond their 

 point of origin. As they are in this way carried up onto 

 the exumbral surface the cartilaginous tissue grows so as 

 to fill the space between the base of the tentacle and the 

 bell-margin, fox-ming a round cushion oi" hard tissue. In 



